1. Field of the Invention
This invention relates to a ground circuit apparatus for electronic equipment and, more specifically, to a ground circuit apparatus for electronic equipment having a plurality of circuit blocks, such as a video tape recorder, a television and audio equipment.
2. Description of the Related Art
In a conventional camera-incorporated video tape recorder (hereafter referred to as a VTR), a camera circuit, a logic circuit, a video circuit and an audio circuit are configured independently and a substrate having those circuits attached thereto is installed on a mount called a mechanical deck. FIG. 1 is a block diagram of a ground circuit apparatus of a conventional camera-incorporated VTR. Power is supplied either from battery output terminal 12 or AC adapter output terminal 14. A DC power of +10V, for example, is supplied by selecting either of the two power sources with switch 16 to DC-DC converter 18 and regulators 20, 22 and 24 through the power line indicated by +B in the diagram. After converting the input DC power to a specified DC voltage, DC-DC converter 18 supplies the power to the individual circuits of signal processing circuit group 36 composed of camera circuit 26, logic circuit 28, servo circuit 30, video circuit 32 and audio circuit 34.
Regulator 20 regulates the input DC power and supplies the power to microcomputer 38. Regulator 22, which serves the motors, regulates the input DC power and supplies it to motor driving circuit group 46 constituting of drum motor 40, loading motor 42 and capstan motor 44. Regulator 24 regulates the input DC power and supplies it to electronic view finder (EVF) 48.
Let GND1 denote a ground line common to camera circuit 26, logic circuit 28, servo circuit 30, video circuit 32 and audio circuit 34 and also a ground circuit for regulator 20 and microcomputer 38 (indicated by the dotted lines in the diagram). Let GND2 denote a ground line for drum motor 40, loading motor 42 and capstan motor 44. Then, GND1 is connected with GND2 through DC-DC converter 18 and regulators 22 and 24. Chassis 50 is connected with the ground line GND1 through the ground line GND3 of video circuit 32. As shown in FIG. 1, there are provided additional ground lines between the adjacent circuits, that is, between camera circuit 26, logic circuit 28, servo circuit 30, video circuit 32, audio circuit 34 and microcomputer 38. Likewise, ground lines are provided between drum motor 40 and loading motor 42, and between loading motor 42 and capstan motor 44.
The ground lines of the two circuit groups 36 and 46 are connected through DC-DC converter 18 and regulators 22 and 24. Within the respective circuit groups, that is, within signal processing circuit group 36 and within motor driving circuit group 46, a loop is formed by the respective ground lines. Two ground lines are sometimes connected, and sometimes not connected for such a reason as electromagnetic induction is more likely to occur by connection. To take an example, when a connection is made directly on the load-applied ends between GND1, the ground line of camera circuit 26, logic circuit 28, servo circuit 30, video circuit 32 and audio circuit 34 in signal processing circuit group 36 and GND2, the ground line of drum motor 40, loading motor 42, and capstan motor 44 in motor driving circuit group 46, a leak of motor noise into signal processing circuit group 36, through the ground line increases because the current following into the motors increases considerably in an instant when motors are started. Thus this adversely affects signal processing circuit group 36, in which video circuit 32 and audio circuit 34 are most affected.
Therefore, as shown in FIG. 1, GND1 and GND2 are generally not connected directly on the load-applied ends. In consequence, the ground line GND1, as well as ground line GND2, is connected directly to chassis 50, and GND1 is connected to the ground line GND2 through DC-DC converter 18, with the result that the ground line route from GND2 to chassis 50 is considerably long.
In some types of VTRs, a portion of the circuit using the ground line GND2 group (including GND2) is sometimes led to the outside of the VTR. Further, since EVF 48 consumes a considerable amount of current, and noise occurs in each horizontal period or vertical period, the power source to EVF 48 is separated from the power source to signal processing circuit group 36 and a ground line of EVF 48 is also provided on the side of the ground line GND2 as illustrated in the diagram. In such a case, the video signal line, the power line and the ground line GND2 are, in some cases, collectively led through an external connection terminal to the outside of the VTR.
In a VTR having such a ground circuit apparatus, when static electricity, accumulated on the human body, for example, is discharged to the ground terminal of EVF 48 of the ground line GND2 group, the charge flows through the ground line GND2, passes through the ground line GND1 connected to DC-DC converter 18 and is discharged through the ground line GND3 of video circuit 32 to chassis 50. However, the ground line route from the ground line GND2 through GND1 and GND3 to chassis 50 is considerably long. Consequently, when the discharge of static electricity is generated, the signal processing block tends to be affected by noises. Therefore, there is a concern that the VTR may malfunction and the ICs in the signal processing block may break down.
To be more specific, when discharge of static electricity is generated on the ground terminal of the EVF of a camera-incorporated VTR which is a part of the long ground line GND2 connected to chassis 50, there is conventionally a possibility that the ICs of the signal processing block connected to the ground line break down or malfunction before the discharged charge passes through the long ground line and is discharged to chassis 50.
To solve those problems, if the ground line GND2 and chassis 50 are connected by a short circuit on the load-applied ends, noise is induced from drum motor 40, loading motor 42 and capstan motor 44 of motor group 36 and EVF 48 through the ground line to camera circuit 26, logic circuit 28, servo circuit 30, video circuit 32 and audio circuit 34 of signal processing circuit group 36 and microcomputer 33. This gives rise to a problem of deterioration of the VTR equipment.